RAJALAKSHMI ENGINEERING COLLEGE

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RAJALAKSHMI ENGINEERING COLLEGE

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UNIT- I

AUTOMATA

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Finite Automata

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States Open Closed Sensors Front – Someone on the Front pad Rear – Someone on the Rear pad Both – Someone on both the pads Neither – No one on either pad

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UNIT - II

REGULAR EXPRESSIONS AND

LANGUAGES

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Questions about Regular Languages

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Question contd…

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Singleton Languages are Regular

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Finite Languages are regular

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Closure Properties for Regular Languages

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Myhill-Nerode Theorem

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UNIT III

CONTEXT-FREE GRAMMAR AND

LANGUAGES

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Context Free Grammars

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Cont…

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Cont…

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Defining CFG

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Notational conventions For CFGs

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OTHER CFG EXAMPLES

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Languages of CFG

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Languages of CFG

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Regular Languages and CFL

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Translating FAs into CFG

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Formalizing the Translation

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Closure Properties of CFL

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Proving CFLs closed under Union

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IDEA

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Formal Construction of Gu

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Derivations

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Sentential Form

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Left Most and Right Most Derivation

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Right-Linear Grammars

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Ambiguous Grammars

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Ambiguous Grammars

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Pushdown Automata

Recall our study of regular languages. They were defined in terms of regular expressions (syntax). We then showed that FAs provide the computational power needed to process them.We would like to mimic this line of development for CFLs. We have a “syntactic” definition of CFLs in terms of CFGs. What kind of computing power is needed to “process” (i.e. recognize) CFLs?Do FAs suffice?

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Cont…

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Cont…

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Example PDA for{0n1n|n≥0}

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Formal Definition

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Instantaneous Description

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Language accepted by PDA

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Equivalence of CFGs and PDAs

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Cont…

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Deterministic PDA

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UNIT - IV

PROPERTIES OF

CONTEXT-FREE LANGUAGES

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Chomsky Normal Form

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Defining CNF

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What is the Big Deal about CNF?

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Cont…

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Converting CFGs into CNF

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Eliminating ε-Productions

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Cont…

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Cont…

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Nullability

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Generating an ε-Production-free CFGs

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Converting CFGs into CNF

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Eliminating Unit Productions

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Cont…

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U(G,A) and New Productions

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Example

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Cont…

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Formal Construction

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Eliminating Terminal Productions

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Example

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Formal Construction

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Pumping Lemma For CFLs

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Cont…

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Derivation Trees

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Defining Derivation Tree

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Example

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Derivation Trees and CNF

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Pumping Lemma for CFL

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Proving Languages Non Context Free using Pumping Lemma

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Prove that L= {aNbNcN|N ≥0} is Not a CFL

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Proof Cont…

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Turing Machines

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A Finite Automaton

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A Pushdown Automaton

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A Turing Machine

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Cont..

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Differences

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Example

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Cont…

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Cont…

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Cont…

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Formal Definition

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Cont..

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Cont…

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Cont…

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Configurations

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Cont…

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More Configuration

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Accepting a Language

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Enumerable Languages

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UNIT – V

UNDECIDABILITY

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Example

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Example

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Example

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Element Distinctness

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Variants

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Multitape Turing Machine

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Equivalence

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Simulation

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Simulation

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Non Deterministic Turing Machine

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Decidability

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Example

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Theorem

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Theorem

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Theorem

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Theorem

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Halting Problem

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Cont..

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Cont..

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Turing Machines are countable

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Cont..

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Post Correspondence Problem

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Cont…

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Cont...

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Cont…

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Cont…

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Cont…

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NP-complete Problem

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Decision Problems

To keep things simple, we will mainly concern ourselves with decision problems. These problems only require a single bit output: ``yes'' and ``no''.

How would you solve the following decision problems?

Is this directed graph acyclic? Is there a spanning tree of this undirected graph with total weight less than w? Does this bipartite graph have a perfect (all nodes matched) matching? Does the pattern p appear as a substring in text t?

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P

P is the set of decision problems that can be solved in worst-case polynomial time:

If the input is of size n, the running time must be O(nk). Note that k can depend on the problem class, but not the particular instance.

All the decision problems mentioned above are in P.

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Nice Puzzle

The class NP (meaning non-deterministic polynomial time) is theset of problems that might appear in a puzzle magazine: ``Nicepuzzle.''

What makes these problems special is that they might be hard tosolve, but a short answer can always be printed in the back, and it is easy to see that the answer iscorrect once you see it.

Example... Does matrix A have an LU decomposition?

No guarantee if answer is ``no''.

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NP

Technically speaking:

A problem is in NP if it has a short accepting certificate. An accepting certificate is something that we can use to quickly show that the answer is ``yes'' (if it is yes). Quickly means in polynomial time. Short means polynomial size.

This means that all problems in P are in NP (since we don't evenneed a certificate to quickly show the answer is ``yes'').

But other problems in NP may not be in P. Given an integer x, is it composite? How do we know this is in NP?

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Good Guessing

Another way of thinking of NP is it is the set of problems that can solved efficiently by a really good guesser.

The guesser essentially picks the accepting certificate out of the air(Non-deterministic Polynomial time). It can then convince itself that it is correct using a polynomialtime algorithm. (Like a right-brain, left-brain sort of thing.)

Clearly this isn't a practically useful characterization: how could we build such a machine?

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Exponential Upperbound

Another useful property of the class NP is that all NP problems canbe solved in exponential time (EXP).

This is because we can always list out all short certificates in exponential time and check all O(2nk) of them.

Thus, P is in NP, and NP is in EXP. Although we know that P isnot equal to EXP, it is possible that NP = P, or EXP, or neither. Frustrating!

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NP-hardness

As we will see, some problems are at least as hard to solve as any problem in NP. We call such problems NP-hard.

How might we argue that problem X is at least as hard (to within a polynomial factor) as problem Y?

If X is at least as hard as Y, how would we expect an algorithm that is able to solve X to behave?

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One of the central (and widely and intensively studied 30 years) problems of (theoretical) computer science is to prove that

(a) PNP (b) NP co-NP.

All evidence indicates that these conjectures are true. Disproving any of these two conjectures would not only be considered truly spectacular, but would also come as a tremendous surprise (with a variety of far-reaching counterintuitive consequences).

NP-complete: Collection Z of problems is NP-complete if (a) it is NP and (b) if polynomial-time algorithm existed for solving problems in Z, then P=NP.

P

NPco-NP

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NP-Complete Problems

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